1. Field of the Invention
The present invention relates to a method of transmitting AM stereo signals and, more particularly, it relates to method and apparatus for AM stereo transmission, excellently compatible with the conventional monaural receivers and with a narrow side-lobe frequency band.
2. Prior Art
Various methods have been proposed and used practically in the United States to achieve AM stereo transmission. None of these methods, however, are ideal from the viewpoint of their compatibility with conventional monaural receivers. One of these methods is to transmit a transmission signal S which is represented as follows: EQU S=(1+L+R) cos (.omega.t+.phi.)
wherein L, R and .omega. represent left audio signal, right audio signal an carrier angular frequency, respectively, and .phi. is the phase angle given by tan.sup.-1 (L-R)/1+L+R. When this signal S is received by the conventional monaural receiver, its envelope (1+L+R) is detected. No distortion can be found in this detected signal, because only the sum signal which represents the monaural signal is present in it. Therefore, this transmission method is complete in its compatibility with the conventional monaural receivers. However, this signal S has such a drawback that a high frequency side-lobe causes the signal to occupy a wide band. FIG. 1A shows a frequency spectrum in a case where only the left signal of 8 KHz is modulated by 80% according to this method, and it is apparent from FIG. 1A that high frequency side-lobe is present.
Another method is to transmit a signal S which is represented as ##EQU1## Since the difference signal is present in the radical sign of this signal envelope, in addition to the sum signal which represents the monaural signal, distortion can be found in this envelope-detected signal. Therefore, this second method is not entirely compatible with the conventional monaural receivers. However, this signal S can be changed as follows: EQU S=(1+L+R) cos .omega.t-(L-R) sin .omega.t.
As is apparent from the above equation, it is composed of two AM waves, and no side-lobe higher than secondary degree is present accordingly. FIG. 1B shows a frequency spectrum in a case where only the left signal of 8 KHz is modulated by 80% according to this second method.
As described above, the first method causes no distortion when the signal is envelope-detected, and becomes excellently compatible with the conventional monaural receivers, but its side lobe causes it to occupy a wide band. On the contrary, the second method maintains the occupied band, as narrow as the conventional monaural AM signal, but it causes distortion when envelope-detected and it is inferior in its compatibility with the conventional monaural receivers. Therefore, neither of these methods is ideal for AM stereo transmission.